Augmented glenoid baseplates - the challenge of seating

Failure of the glenoid baseplate is an important complication of reverse total shoulder arthroplasty (see link and link).

At least two factors are important in minimizing this risk: (1) screw fixation in quality bone and (2) seating of the baseplate on the prepared glenoid.

Excellent seating is achieved when the glenoid is reamed to a single concavity and fit precisely with a convex backed baseplate resulting in full contact. 

In cases where the glenoid is biconcave or posteriorly or superiorly deficient to the extent that it cannot be safely reamed to a single concavity, 

the surgeon may elect an augmented baseplate.  

Partial and full augments are available.

Preparing the glenoid bone for an augmented component is more complex than for a baseplate with a single backside convexity.

Fitting the augmented component to the prepared bone is also more complex. See Rocking Horse Loosening of the Baseplate in Reverse Total Shoulder Arthroplasty.

 The preparation may require two different reaming steps 

And then positioning the baseplate to exactly fit the the prepared glenoid.

A recent article examined the Effects of implant rotational malposition on contact surface area after implantation of the augmented glenoid baseplate in the setting of glenoid bone loss

The authors point out that the backside of the augmented glenoid baseplate is not perpendicular to the axis of the central post/screw. Thus, if the baseplate is implanted with any rotational malposition, this could affect the backside contact area with loss of stability and the potential for bony ingrowth. 

They assessed the effect of rotational malpositioning of a full-wedge augmented on glenoid implant backside contact area using synthetic scapulas and a 15° full-wedge glenoid baseplate. 

The contact pressure between the baseplate and the glenoid surface at rotational positions 5°, 10°, and 15° clockwise (CW) and counterclockwise (CCW) from the central axis was measured with Extreme Low Fujifilm Prescale (Tekscan).

Gross contact was evaluated with a computed tomography scan.

The average contact area at zero degrees of malrotation was 37%. The average contact areas for the simulated malposition cases were

14% at 15° CCW,
25% at 10° CW,
19% at 15° CW.

On computed tomography, at 15° CCW, the contact area decreased by 39%; at 15° CW, the contact area decreased by 38%.

CT scans and Fujifilm pressure contact film for baseplates implanted at 0°, 10°, and 15° clockwise demonstrating decreasing amounts of surface contact between the baseplate backside and the glenoid surface with increasing rotational malposition

Comment: Awareness of the challenges of preparing for and fitting of an augmented glenoids should minimize the risk of failure of these components.

Comments welcome at shoulderarthritis@uw.edu

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Here are some videos that are of shoulder interest
Shoulder arthritis - what you need to know (see this link).
How to x-ray the shoulder (see this link).
The ream and run procedure (see this link).
The total shoulder arthroplasty (see this link).
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
The smooth and move procedure for irreparable rotator cuff tears (see this link).

Shoulder rehabilitation exercises (see this link).